Starships: The Problem of Arrival

by Paul Gilster on March 16, 2012

You wouldn’t think that slowing down a starship would be the subject of a totally engrossing novel, but that’s the plot device in Poul Anderson’s Tau Zero (1970, though based on a 1967 short story called “To Outlive Eternity”). Anderson’s ramscoop starship, the Leonora Christine, can’t slow down because of damage suffered in mid-cruise. Edging ever closer to the speed of light, the crew experiences all sorts of time dilation wonders as they wrestle to regain control, and the ending, while scientifically dubious, is also in every way unforgettable. Anderson could be guilty of over-writing but few writers are gifted with his sheer imaginative sweep.

I’m thinking that coupling a ramscoop with a problem in deceleration is just the ticket for getting into the whole issue of starship arrivals. We can start with Robert Bussard’s 1960 paper “Galactic Matter and Interstellar Spaceflight,” which unwittingly paved the way for the whole magsail concept. Bussard came up with what for a time appeared to be the ultimate in fast transportation, a ramjet that collected interstellar hydrogen in an electromagnetic scoop thousands of kilometers in diameter as it traveled. The collected hydrogen would be used as fuel for the same kind of proton/proton fusion that powers up the Sun. Moreover, this would be an engine that would become more efficient the faster the ship went.

The Ramscoop and the Details

No wonder writers like Anderson and Larry Niven loved the idea, which fed their plots of humanity expanding into the galaxy — even Carl Sagan would point to the Bussard ramjet as a propulsion system that could get us up to a substantial percentage of the speed of light. But in the case of the ramjet, at least one of the devils in the details turned out to be bremsstrahlung radiation, which is produced when charged particles decelerate. Thomas Heppenheimer went to work on this in 1978 and found the Bussard design unworkable because the power that could be produced was dwarfed by the losses from the bremsstrahlung process.

Image: The Bussard ramjet, a concept which may turn out to have more applicability in braking than acceleration. Credit: Adrian Mann.

This wasn’t the end of the ramjet concept because Daniel Whitmire was able to figure out a different way to power up the engine (and in the future we’ll have to talk about Whitmire and his study of the CNO cycle, which makes for a much more powerful and efficient fusion engine), but the most troublesome critique emerged in 1985 through the work of Dana Andrews and Robert Zubrin. Bussard assumed an electromagnetic scoop of vast proportions, but Andrews and Zubrin came to realize that such a scoop produced more drag than it did thrust. In fact, their work showed that when leaving a star system, a ramscoop could serve as an electromagnetic sail. Thus the magsail entered into the lexicon and we began pondering its uses.

If we can’t use Bussard ramjet techniques in cruise, why not use them upon arrival? Switch on the magsail as the vehicle approaches its destination solar system and let it use the star’s stellar wind to brake against. The magsail produces efficient deceleration at high velocities, but an incoming spacecraft could also deploy a conventional solar sail upon arrival for final braking and movement within the planetary system. Magsails could also be used to provide the mission’s initial acceleration, pushed by a particle beam or by a stream of incoming pellets turned into plasma — Gerald Nordley has explored this concept and others germane to our purposes here.

Hybrid Starship Designs

Now we’re talking hybrid approaches to interstellar propulsion. A spacecraft might achieve its initial acceleration, for example, through other forms of fusion, or perhaps through beamed microwave or laser sail technologies, while deploying the magsail for arrival. Get into the details and hybrid systems begin to make sense, because there is nothing that says you have to use the same technologies for interstellar braking as you do for the rest of the journey. But the great problem of deceleration looms over the entire topic of interstellar travel, and it behooves us to think of ways to take advantage of external braking possibilities wherever possible, unless we want to devote most of the bulk of our spacecraft to an onboard deceleration system and its fuel.

I was interested to see that John Mathews considered the deceleration question in his recent paper on self-replicating spacecraft (see Robotic Networks Among the Stars and the subsequent three entries here). Mathews is well acquainted with magsail concepts and advocates braking against the stellar wind, noting that solar sails are efficient only relatively close to the star. Where he moves us a step forward is in his idea of using electrodynamic tether technologies to generate huge amounts of energy from the spacecraft’s movement through the stellar wind, powering the magsail itself and offering options for driving other devices.

The comparison of solar to magnetic sail in terms of decelerating a spacecraft is one we have to get into at more depth, so we’ll continue the deceleration discussion on Monday and most of next week, for it turns out that magsail and solar sail braking are only two of the options we might consider. I want to go through all of these and offer some references for an area of the interstellar conundrum that doesn’t often get the attention it deserves. Project Icarus has been pondering deceleration options too, and I’ll use some of our synergy with their work to consider what might be done.

The Bussard paper mentioned above is “Galactic Matter and Interstellar Spaceflight,” Astronautica Acta 6 (1960), pp. 179-194. Andrews and Zubrin’s key paper on the Bussard ramjet and drag is “Magnetic Sails and Interstellar Travel,” International Astronautical Federation Paper IAF-88-5533 (Bangalore, India, October 1988). Or you can read Zubrin’s well written exposition of all this in Entering Space: Creating a Spacefaring Civilization (New York: Tarcher/Putnam, 1999). This one should be on your shelf in any case.

At risk of stating the obvious, the higher the speed of the interstellar craft, the greater the deceleration issue. So, a slower speed mission with only a survival-of-humanity rationale should have less deceleration issues than a science rationale mission requiring speeds at significant percentages of the speed of light.

I think no one has yet explored the obvious place to decelerate — at the bow front of the moving star. Bow shocks form at distances of ~100 AU from the star, offering a bumper of higher plasma and gas density, a heightened spectrum of plasma waves, etc. Skating along the parabolic bow profile gives an incoming craft a long trajectory counter to the star’s motion, a greater yield in both deceleration and energy harvesting (as in Mathews’ paper). Turning the craft through the v x B force, as Forward first argued, will allow it to maneuver in the star’s magnetosphere, too, shedding velocity as it goes.
The most skillful interstellar sailing comes, then, at the end of the cruise.

I should clarify that an incoming starship can spiral in along the bow shock paraboloid surface, using v x B and staying in the high density plasma stacked up along that surface. This gives a far longer working trajectory for deceleration.

I love the hybrid concept. If the acceleration mechanism is rocketry, a hybrid design will have a mass ratio the square root of the mass ratio of a pure rocketry mission. This is very significant for allowing a higher mass ratio for acceleration and higher cruise velocity.

@Dr. Benford – excellent suggestion about using the bow front to decelerate – hopefully the Voyagers will keep sending us data about this region of Sol for years to come.

PS: I cannot find a web reference now, but I recall that decades ago Bussard himself had admitted he was aware of a grave math error in his 1960 paper even shortly after publication – he had assumed the σ (cross section) of the rate limiting step (P + P → D + positron + electron neutrino + 0.42 MeV) of the proton–proton chain reaction was many orders of magnitude more favourable than the true figure. This error was completely independent of the issue of the bremsstrahlung losses, which were calculated much later.

I find this interesting, as Larry Niven’s great popularization of the Bussard ramjet drive in SF occurred years after Bussard himself knew the idea was a nonstarter. I wonder if Niven knew back then? I enjoyed Larry’s stories immensely, but if he was popularizing a known falsehood that was a bit naughty. We knew that the General Products hull, the stasis box, the Puppeteers, and the Sinclair molecule were fantasies. But many young readers were led to believe that the Bussard ramjet was possible under the laws of our universe.

Why is this an issue? In my opinion, one of the reasons that Apollo was quickly greeted with a collective yawn by humanity, is that SF had promised so much more and soon too. So support for real space travel has withered while theatres can still be packed for SF fantasies with the latest CGI. And far more people today keep up with the Kardashians and the Cardassians than with Rusty Schweickart or Franklin Chang-Díaz. Sad.

Dr. Benford: This sounds really good, but could it be quantified? In my intuition (which, of course, may well be wrong) the density of plasma/gas at that distance is really, REALLY low, bow shock or not. So are both the B and the Q part of the v x B force (Q being the charge needed for there to be a Lorentz force). If you do the actual math, what are the kinds of delta-V that one could hope to get in a few hundred AU of nearly empty space with the very weak solar or interstellar magnetic fields? What velocity would that allow one to decelerate from?

I suspect the numbers would be very unfavorable, but I would love to be proven wrong.

Ramjets aren’t yet a non-starter, as Whitmire’s CNO-Cycle fusion is one example of the options available to us – used to good effect in Greg’s Galactic Centre series. His description of the “Lancer” in “Across the Sea of Stars” got me interested in the idea. But ramjets won’t be easy. Bussard knew that, but insisted that CNO catalysis, or some other fusion trick, would eventually make them able to reach 0.999999 c or so. Tom Ligon talked with Bussard often about the idea, right up to his death in 2007.

Personally I like Alastair Reynolds’ version which used GUT monopoles to catalyse proton-decay and get even higher performance. Or the slightly more mundane version which uses antimatter to inject energy into the mass-stream, thus making a high-powered Ram-Augmented Interstellar Rocket. The future’s possibilities aren’t all known yet.

Poul Anderson’s ripping yarn , Tau Zero, rose up and smacked me my first year in graduate school when I started my PhD. I had been interested in Bussard Ram Jet since I had seen the original paper paper ten years before in 1960. I am guessing, but pretty sure, Anderson had come across Sagan’s exposition about constant acceleration relativistic starflight , probably in Intelligent Life in the Universe ( Shklovski, I.S and Carl Sagan. Intelligent Life in the Universe. San Francisco: Holden-Day, 1966). Sagan does not explicitly state it there but he did in other publications , that one could circumnavigate the universe (at one g) in a human lifetime. Back in those days standard cosmology took it that the universe was closed. Sagan may have discovered this for himself but Eugen Sänger was the first, as far as I know, to calculate this in his paper Zur Flugmechanik der Photonenraketen. Astronautica Acta 3 (1957).
Anderson put two and two together and-produced that trump-them-all problem solving story (the SF John Campbell loved so much, tho the short story was published in Galaxy).
The standard cosmology in the literature of the 1960’s was that the universe was closed , though by 1970 observational astronomy was starting to close in on ‘flat’ and open. When Anderson wrote the novel I think the only model for the ‘big crunch’ was Gamow’s Yelm. All that has changed with inflation cosmology and now the accelerating universe.
Even in 1970 , to calculate the flight dynamics of the Leonora Christine one would of had to switch from Special Relativity to General Relativity for the latter part of the flight. It’s would be a good exercise problem for some grad student out there to calculate the flight dynamics of that ship in an expanding accelerating universe.
I don’t think enough was known when Anderson wrote the novel, but the Leonora Christine could probably have been decelerated by finding a galactic nucleus black hole , make a close encounter , orbit it, shed energy by gravitational radiation. Inside 10 Schwarzschild radii one can do loop-d-loop orbits (really non-Newtonian!)) and still escape. A spinning big black hole has an even stranger orbit regime.
Of course one would have to have some super science shielding , but heck what is super science for!

Could we not use the final spent stage of the fusion rockets exhaust (still firing-just a lot less) to give a deceleration force to a magsail/sail. The decelaration on a small probe would be very high as it catches the partical stream.

Dr. Robert Bussard.
I had dinner with Bob Bussard at an AIAA meeting in San Francisco in 1979.
Poul Anderson was at the same table too, I am not sure, but I think that is the only time they met.
Anderson was engaged in conversation with the other people at the table all a-thrill to have the famous SF author there. That left me an Bob alone, which was fine with me.
Dr. Robert Bussard (PhD Princeton) was one of the most impressive men I have ever met. A tall rangy guy, friendly, with great sense of humor and as sharp a mind as I have ever seen. Bussard also had a gravitas about him, he radiated a persona of a man who walked the halls of power.
He knew that one of my best friends and my co-author was Dan Whitmire, so we had corresponded a little.
I wish I kept notes or a diary but I don’t , here’s is what I remember. I asked him about the genesis or the interstellar ramjet. He said he had been interested in spaceflight as a kid, he was an SF reader then, not so much in later life.
He had started working on nuclear propulsion in 1953 at Oak Ridge and then moved to Los Alamos in 1955 to work on Kiwi (Rover-A). He worked intensely on that project. He and R. DeLauer produced the first monograph (as far as I know) on nuclear rocket propulsion in 1958.
Interstellar flight had always been on the back of his mind , fission and fusion propulsion had been in the air, as a method for starflight, for years. He fretted about the mass ratio problem for interstellar flight. Some documents from Lawrence Livermore National Laboratory passed by him one day about project Pluto (a nuclear ramjet, known as the flying crowbar, I’ll let you all look that one up).
He told me that at breakfast at Los Alamos one day having huevos rancheros, which came with corn tortillas, he was eating them southwestern style , that is rolled up into a cylinder… when bingo! it hit him. Scoop interstellar hydrogen and fuse it. (I think this story is true, I read him repeating it again except he said dinner, no matter.) That story stuck with me all these years.
I wish I could remember what else we talked about, it must have for about 3 hours , with wine, I remember him being a gem crack raconteur.
I don’t think Bob wrote another technical paper about interstellar flight.
Bussard remained interested in interstellar flight the rest of life, he wrote and spoke about it … but his real focus from the late 60’s onward was fusion.
He had some very original ideas but fought a losing battle from the 70’s to the 2000’s to get them funded.
Bob Bussard was a singular physicist.

The great thing about mag sails is that they are scalable, and there is no reason we couldn’t be experimenting with them now. Perhaps a worthy project for DARPA to fund a discovery class mag sail mission?

@qrall
I have read Whitmire and there is less substance to his CNO concept than some of the (highly aromatic) late night sophomore bull sessions I attended at Caltech in the same era. This cycle dominates in high mass stars, but there are no credible ideas as to how this could be done in a spaceship combustion chamber without the loss of the massive ions used as the catalyst.

Of note, the bremsstrahlung losses of a ramscoop have been calculated to exceed the fusion thrust obtainable by circa 10^9! Over the decades various folks have tried to rescue the scoop as something other that the excellent brake that it well could be. There are some RAIR concepts archived on the web from the mid 1990s “Lunar Institute of Technology” project which involve using a particle accelerator to position a concentrated propellant (not fuel) stream ahead of the craft, which could be collected with a small scoop at a rather small relative velocity. You can find the “explorer” class ship concept with google.

What can be done about avoiding collisions with interstellar comets, rogue planets, brown dwarfs and black holes?

Some of those could be used for gravitational slingshots. But how fast can we go past a planet or whatever, and still be able to use it for a gravitational slingshot? How small a mass would be worth using?

How’s this? A starship encounters a brown dwarf. The ship decelerates enough for the crew to study it, then they dismantle it (somehow! using futuristic tech). They then use it for fuel to accelerate toward their intended destination–until the next time they encounter a brown dwarf or something.

Negative mass repels positive mass, so it could repel the interstellar debris which might damage the ship. But by what angle a, would it divert a body of mass m, when the ship is traveling at velocity v? I have no idea.

Slowing down the starship is more than half the battle, especially if your starship is propelled by a rocket of some sort. Essentially, a pure rocket starship must accelerate all the fuel it needs to slow down, so you aren’t just launching a rocket powerful enough to reach interstellar cruising speeds- you are launch a rocket that carries another rocket capable of accelerating from a good fraction of light-speed down to zero!!

Naturally, this raises hell with your mass ratio- you have to carry more propellent just to carry the propellent you need to slow down, so you end up with the planet Jupiter in tow. Hybrid propulsion is an excellent way to overcome this problem. A hybrid rocket-magsail starship can slow down on “empty” after using up all its propellent.

This is what I like to call “Cosmic Freeloading”- using objects in space to your advantage. Instead of slowing down with rockets, you can use aerobraking or magsails. Instead of carry all the propellent you need to get home, you can send the enlisted men out to shovel whatever they can find into the fuel tank- I mean practice “in-situ resource utilization”.

None of the “freeloading” techniques work for velocities greater then a few hundred km/s. Stellar magnetic fields and stellar wind or radiation are too weak and too local to be of much use. The ISM and interstellar fields are too thin to take up momentum of any significance. Gravitational slingshots are only good for delta-v on the order of escape velocity, pitiful compared with our desired cruise velocity.

On the other hand, all it takes to brake down with a rocket of given mass ratio is to reduce the cruise velocity by half. Thus, all things considered, this last method is almost certainly the one of choice.

We need not worry about collisions with objects, such collisions are astronomically improbable. Using them as waystations intentionally is pretty much out of the question: a) Anything much smaller than a brown dwarf will be undetectable from lightyears away, and b) the energy budget required to stop and take off again is enormous and unlikely to be worth anything that we could expect to find at any such place.

@stephen
Long ago Freeman Dyson wrote a short paper about using binary system to extract energy.
(Dyson, Freeman J. “Gravitational Machines.” Interstellar Communication, A. G. W. Cameron, Editor, New York: Benjamin Press, 1963, Chapter 12. ).
It is the same physics that JPL uses for gravitational asset trajectories. Dyson considered neutron star binaries, but I think later black hole binaries. It is possible to extract a considerable ‘boost’ from such an encounter. It works the other way too, pick the right crossing direction and one slows down. (For black holes there is some GR physics that gives one a little more umph! than the Newtonian physics does.)
A lot of caveats here , have to find neutron star and black hole systems that are long enough lived against decay by gravitational radiation to be useful.
I don’t think we know the frequency of occurrence of neutron star and black hole binaries , it may not be very big, so not very useful. One has to calculate the ‘impact parameter’ of the encounter but usually one does not approach close enough so that tidal forces tear one up. It’s a great idea but is probably impracticable as a boost or deceleration mechanism.
More interesting is rotating black holes, the orbits in Kerr-spacetime are downright weird. Frame dragging might be used for both boost and stopping.
The whole zoo of possible trajectories in Kerr-space time has , as far as I know, not been totally classified. Rotating black holes are probably more prevalent than non-rotating ones (probably more than black hole binaries). Besides getting there it would take one hell of a guidance, control and navigation system to pull off the encounter without killing yourself!

Let’s say you want to accelerate your ship to 0.1c – and the decelerate it at the target solar system from 0.1c to 0.

First, you use a mass driver to accelerate all the (fusion or fission) fuel the ship needs to ONLY decelerate from 0.1c to 0, on the future flight path of the ship.
Then, you launch the ship, carrying ONLY the fuel it needs to accelerate to 0.1c.

The ship will accelerate to 0.1c using the on-board fuel. Then the ship will use a very small ramscoop (a few tens of meters across) to pick up the fuel you sent via mass driver. After that, the ship will cruise until it is time to decelerate, an endeavor for which it now has the fuel.

In conclusion, you will need only 2 x the fuel needed for acceleration to accelerate+decelerate. You will not have to deal with the exponential increase requested by the rocket equation.

The problems with this proposal:
First, the mass driver will need to be VERY LONG to accelerate the fuel to 0.1c (compared to other constructs proposed for interstellar flight, though, it’s of a modest enough size). This is why it’s recommendable to make the individual fuel pellets very light – a few grams.
Second, you need to target the fuel pellets precisely. This can be accomplished by having correction stations positioned around the flightpath of the fuel pellets (at some distance from the mass driver); these will correct any deviationn from the desired trajectory of the fuel pellets.

I musk ask this question even though I suspect it naive. What is the difference between the deceleration methods used here and an Alfven engine?

I recall it being claimed that this method was almost up to the task of interstellar breaking. I realise that the implied rate of decelerations is much less than those mentioned here, but I can‘t help wondering if anything analogous to the bow shock mentioned by Benford can assist on much larger scales than 100AU?

Way before Heppenheimer , Andrews and Zubrin’s papers on limitations of the interstellar ramjet a student of Phillip Morrison’s MIT published on this in 1969:
J. F. Fishback. Relativistic interstellar spaceflight. Astronautica Acta 15, 25-35, 1969.
That J.F. stands for John Ford.
This is gem of a paper. The main part of it deals with the stress on an ISR’s magnetic field source. Fishback shows that material properties limit the distances of an ISR to 10,000 light years before ship’s magnetic field source busts. I did not consider 10,000 ly too bad a start!
Fishback may have been the first to consider bremsstrahlung, and synchrotron loses, he has a long section on radiation loses.
Fishback moved to U. Cal. Berkley to do graduate work. Once I tried to find him but he had vanished. Saw Phillip Morrison in the late 70’s and asked what had happened, he told me Fishback has committed suicide.

“None of the “freeloading” techniques work for velocities greater then a few hundred km/s. Stellar magnetic fields and stellar wind or radiation are too weak and too local to be of much use. The ISM and interstellar fields are too thin to take up momentum of any significance.”

-Eniac

Actually, you are wrong- a magsail’s deceleration efficiency drops off as the starship slows down, at least according to The Starflight Handbook. Please note that magnetic sails don’t use solar or galactic magnetic fields- they ionize incoming hydrogen atoms and deflect them in a large magnetic field to produce drag. I am not sure how much drag such a system can produce, but I know that no researcher has suggested that magnetic sails are not useful as braking devices. After all, it was the synchotron drag problem that doomed the Bussard ramjet!!

At high relativistic velocities, the stray hydrogen atoms in interstellar space become a deadly hail of induced cosmic rays, making a good case for streamlining starships and inspiring would-be galactic explorers to try to find a way to put them to use. Apparently the dream of ion scoops lives on, as Icarus Interstellar just started a project to examine interstellar ramjets.

There is also the RAIR, or ramjet assisted interstellar rocket, which scoops up hydrogen and accelerates it using energy from fuel stored onboard. RAIR’s have unlimited propellent, but can only carry a limited supply of fuel, so they can’t continue to fire indefinitely like a fusion ramjet can.

What if, instead of decelerating the entire ship, we could catapult a small part of the ship into the opposite direction? Basically you’d need a part of the ship to be a fully operational ship of its own, and a mechanism to catapult it away from the rest of the mother ship.

Effectively such a maneuvre would be like using the entire ship’s mass (minus that much smaller landing craft) and all the power reserves it might have for deceleration.

Obviously such a scheme would only be useful for a colonization mission, as the smaller, decelerated craft would then land on a planet and establish a colony there, but were unable to return to the place where the journey had started.
And the remaining hulk of the mothership would then move off into the distance, out of the destination system again. It might then even act as an interstellar probe, gathering data and sending it back to the new colony (and the original home system) as it move through the galaxy for ages.

“Interstellar flight had always been on the back of his mind , fission and fusion propulsion had been in the air, as a method for starflight, for years. He fretted about the mass ratio problem for interstellar flight. Some documents from Lawrence Livermore National Laboratory passed by him one day about project Pluto (a nuclear ramjet, known as the flying crowbar, I’ll let you all look that one up).”

One really scary aspect of an already scary Cold War weapon was that once Pluto deposited all of its cargo on the enemy, it could still keep flying around spraying radiation from its engines on other enemy targets.

One of the companies that helped make the test versions of Pluto was called Coors, who later went on to make another kind of product that was just as good at destroying brain cells.

To quote from the above article:

Because of its combination of high speed and low altitude, Pluto promised to get through to targets that manned bombers and even ballistic missiles might not be able to reach. What weaponeers call “robustness” was another important advantage. “Pluto was about as durable as a bucket of rocks,” says one who worked on the project. It was because of the missile’s low complexity and high durability that physicist Ted Merkle, the project’s director, called it “the flying crowbar.”

@ljk
My master’s thesis adviser worked on the nuclear ramjet at Livermore before returning to academia.
He told me the biggest problem they worked on was how to test the damn thing!
They realized that if you could test fly it and land it you could not get within a mile of the thing because it would be so radioactive. One thought to fly the ‘crowbar’ around in circles in the Pacific and let it fall in the ocean.
He said it ever used as a weapon you may not know where was but sure would know where it had been!
Two prototypes were built and even test run, but the Pentagon finally consider the weapon was too freakish they killed it.

The article I linked to above on Project Pluto said, in essence, that the missile would have fried any friends in its flight path as well as foes, making new enemies in the process. The piece also described how Pluto would be flying so low to avoid radar detection that its unshielded (!) nuclear reactor would have literally fried chickens in any barnyard it passed over – not to mention just about any other critter along the way.

You know a nuclear military weapon had to be *really* scary to the generals of the day in an era that seriously considered nuclear-powered automobiles for the populace (the Ford Neutron – 5,000 miles before needing to replace the fuel rod at any roadside station) and making a nice circular ship harbor in Alaska with a nuclear bomb.

Still, I think some form of nuclear power holds a lot of promise (and a better and sooner chance of happening) for interstellar travel. I also think that in the process of making a nuclear propulsion drive for such vessels that we would also come up with a way to create a viable source of power for human civilization. Talk about a space spinoff.

I recall an idea I heard about using a probe built exactly like the Pluto missile to explore the Jovian atmosphere. Practically unlimited range, supersonic speed, and radiation that would make nary a dent in what is already there….

“I recall an idea I heard about using a probe built exactly like the Pluto missile to explore the Jovian atmosphere. Practically unlimited range, supersonic speed, and radiation that would make nary a dent in what is already there….”

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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